The present invention relates to an anti-skid brake control system for a wheeled vehicle and particularly to an arrangement wherein a wheel sensor generated alternating voltage that varies in frequency with the angular velocity of the wheel is utilized to obtain a digital wheel speed signal.
A digital wheel speed signal can be obtained directly from the wheel generated alternating voltage by shaping the waveform and feeding the resulting pulsed output to a binary counter during a predetermined constant time interval. A disadvantage of this method of forming the digital wheel speed signal is the inability to provide the degree of anit-skid control sensitivity desired, particularly at low wheel speeds, due to the constant time interval during which the wheel speed pulses are counted. The speed signals of successive time intervals are compared to detect a change of wheel speed for the purpose of detecting an impending wheel skid condition. Obviously, at low speeds only a relatively few pulses occur during the time interval, whereas at high speeds numerous pulses occur. Consequently, a slight change in speed at high wheel velocities may be detected by a difference of perhaps only a few pulses, while at low wheel speeds this same change in speed will be insufficient to produce a different pulse count and is accordingly undetectable.
This disadvantage of a constant time interval, as mentioned above, can be overcome by providing a time period that varies according to the wheel velocity, during which time period pulses generated at a constant frequency are counted. The wheel speed dependent time period, in an arrangement of this type, is typically formed from a half cycle of the wheel generated alternating voltage waveform. The main disadvantage of such an arrangement, however, is that the impulses counted vary inversely with wheel velocity, thus requiring additional circuitry to reverse the relationship between the pulse count and wheel velocity to obtain a suitable wheel speed signal.
One method of reversing this inverse relationship is to perform an arithmetic operation in which the digital wheel speed signal is subtracted from a constant. Inasmuch as this operation is performed within the duration of a half cycle of the wheel generated alternating voltage waveform, which varies with wheel speed, it will be appreciated that the speed at which the arithmetic operation takes place is critical.